The human eye is like a camera, with the cornea and crystalline lens comprising the lens system and with the retina comprising the film on which the image is formed. If the eye has no refractive error--emmetropia--parallel rays of light entering the eye are focused exactly on the macula in the center of the retina, the area of clearest vision. Some people are indeed fortunate enough to have eyes which at least approximate an emmetropic condition. The majority of human eyes, however, are ametropic. That is, light, and images, are not focused exactly on the retina owing to some abnormality of the refracting mechanism.
Hypermetropia, or farsightedness, is a form of ametropia in which light, or images, nearer than a certain distance, cannot be focused properly on the retina, but rather, are focused behind it. Hypermetropia is corrected by use of a positive lens--typically a convex meniscus.
Myopia, or nearsightedness, is a form of ametropia, in which objects further than a short distance away are not focused on the retina, but rather, are focused in front of it. The so-called "far-point" (the farthest point at which the eye can focus objects) is rather close instead of being at infinity, as it is for the emmetropic eye. At the same time the "near-point" (the nearest point at which the eye can focus objects) is rather closer than for the emmetropic eye. Myopia is corrected by use of negative lenses--typically a concave meniscus.
Positive and negative lens corrections are achieved by the use of lenses having spherical surfaces. Even though a lathe attachment embodying the present invention can be operated to produce lenses having wholly spherical refractive correction, its primary advantage is in being able to provide non-spherical lens surfaces, as required to correct for astigmatism, with equal facility to the production of the relatively uncomplicated spherical lenses.
Astigmatism is a refractive abnormality in which there is a simultaneous difference of curvature in the meridians of the lens mechanism in the same eye. As a result of astigmatism light rays are not focused on the retina as points but as lines of points, and vision is blurred. Astigmatism may be "simple" in that it may be the sole refractive abnormality. More commonly, however, astigmatism may coexist with another refractive abnormality. If astigmatism occurs in conjunction with nearsightedness, a person has myopic astigmatism; if astigmatism occurs in conjunction with farsightedness, a person has hyperoptic astigmatism. Additionally, it should be recognized that mixed astigmatism may exist. That is, the eye may be farsighted in one meridian and be nearsighted in the opposite meridian. The two meridians are orthogonal, but they are not necessarily aligned with the horizontal and vertical planes of the eye.
Astigmatism is corrected by the use of cyclindrical, rather than spherical, correction. Whereas a lens providing spherical correction has a single focal point, a lens providing cylindrical correction has two focal points. This occurs because lenses providing cylindrical correction have a first radius of curvature in one meridian and a second radius of curvature in the second meridian.
If one generates a geometric shape, for example, by revolving a circle about a coplanar line outside the circle, the result is a torus. A doughnut, or a tire innertube, are examples of such a torus. If one then removes a small section from the radially outermost portion of the torus, that section will have two radii of curvature. Along one meridian the section will have a radius of curvature equal to the radius of the circle that was revolved for generating the torus, and along the orthogonal meridian the section will have a radius of curvature equal to the radius measured from the coplanar line about which the circle was revolved to the radially outermost extent of the circle. Such a section will then be designated as having a "toric" configuration, and a lens which provides cylindrical correction, because it similarly has two different-radii of curvature along its orthogonal meridians, can also be designated as a toric lens.
The orientation of the orthogonal meridians of the lens into congruence with the refractive correction required for the eye is directly related to the "axis" of the lens prescription. The axis is designated as the plane of the spherical refractive correction--that is, the angle measured from a horizontal reference plane to the meridian of the most plus power correction. The plane of the most plus power correction--the spherical refractive correction--may coincide with the horizontal plane of the eye itself, but it does not generally do so. As such, an orientation system has been established whereby the horizontal plane of the eye is designated, in the mathematical style, as the 0.degree.-180.degree. plane. The vertical plane of the eye is similarly mathematically designated as the 90.degree.-270.degree. plane. The axis of the lens is thus referenced as the angle measured by the aforesaid convention to the most plus corrective power. The most common axes are 0.degree., 10.degree., 20.degree., 70.degree., 80.degree., 90.degree., 100.degree., 110.degree., 160.degree. and 170.degree..
The second radius, or the cylinder, of the lens is at 90.degree. to the spherical correction.
With this background, then, the prescription required to establish the refractive correction for an astigmatic eye constitutes three parts: the spherical correction, the cylinder and the axis. The radius required to provide the diopter correction for the spherical correction is mathematically determined, and by algebraically adding the cylinder to the spherical correction, the orthogonal radius is also mathematically determined. The orientation of these two orthogonal radii of curvature are then determined by the axis.
The grinding techniques employed to impart the properly oriented two radii of curvature to a standard lens were not really adaptable to the making of contact lenses, and new techniques were developed. One of the most widely employed techniques to create a toric contact lens is to grind, or lathe-cut, and polish the spherical correction onto the concave inner surface of the lens. Thereafter, the rim of the lens is firmly grasped by jaws that impart diametrically opposed forces to the lens. The opposed forces so applied oblate the lens about the locations where the opposed forces are applied. This oblate distortion is then maintained while a second spherical surface of the radius required for the cylindrical correction is ground, or lathe-cut, and polished onto the convex outer surface of the oblated lens. When the distorting force is removed, the lens returns to its original configuration, but the convex outer surface assumes the required toricity to effect refractive correction for astigmatism.
The aforesaid "crimping" process was developed for the so-called "hard" contact lenses and has been successfully adapted for the "soft" contact lenses, as well. However, this crimping method is highly labor intensive and requires a major capital investment for the accurate equipment required to perform the various steps for making such lenses.
There are certain other grinding and/or lap polishing methods that have heretofore been employed to achieve toricity, but all have rather serious drawbacks. For example, two of the better known grinding techniques can only impart toricity to the concave surface of the lens. This is not overly desirable in that better eye contact is achieved if the eye contacting surface can remain spherical.
One other known grinding arrangement has been employed to impart toricity to the convex outer surface of the lens, but this approach can only do so to the entire convex surface, and this can destroy the prismatic ballast employed to orient the lens properly onto the eye.